1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus, and more particularly to a method and an apparatus adapted for exposing a photosensitive substrate to light with a minute circuit pattern so as to manufacture devices of varied kinds, including semiconductor chips such as an IC and an LSI, display elements such as a liquid crystal panel, detection elements such as a magnetic head, and image pickup elements such as a CCD.
2. Description of Related Art
In manufacturing devices such as an IC, an LSI, a liquid crystal panel, etc., by using photolithography techniques, a projection exposure method or a projection exposure apparatus is used to project, through a projection optical system, a circuit pattern formed on a photomask, a reticle or the like (hereinafter referred to as a mask) onto a photosensitive substrate, such as a silicon wafer or a glass plate, coated with a photoresist (hereinafter referred to as a wafer) and to transfer the circuit pattern to the wafer (i.e., expose the wafer to light with the circuit pattern).
To meet a recent trend of increasing the degree of integration of the above-stated devices, a circuit pattern to be transferred to the wafer is required to be more finely and minutely prepared. In other words, it is desired to have a higher resolution and to permit an increase of the area of one chip on the wafer. Hence, the projection exposure method or the projection exposure apparatus which plays a main role in the art of accomplishing minute work on the wafer is being developed these days to increase the resolution and the exposure area in such a way as to form the image of the circuit pattern at a line width not greater than 0.5 xcexcm over a larger range.
FIG. 39 schematically illustrates the arrangement of a projection exposure apparatus conventionally employed. The illustration of FIG. 39 includes an excimer laser 191 which is a light source used for a far ultraviolet ray exposure, an illumination optical system 192, illumination light 193 radiated from the illumination optical system 192, a mask 194, object-side exposure light coming through the mask 194 to be incident on a projection optical system 196, the projection optical system 196 which is a demagnification exposure type optical system, image-side exposure light 197 coming from the projection optical system 196 to be incident on a substrate 198, the substrate 198 which is a wafer as a photosensitive substrate, and a substrate stage 199 arranged to hold the wafer (substrate) 198.
A laser beam emitted from the excimer laser 191 is led to the illumination optical system 192 through delivery optics (190a and 190b). The laser beam is adjusted by the illumination optical system 192 to be converted into the illumination light 193 which has a light intensity distribution, a luminance distribution, an aperture angle (numerical aperture NA), etc., which are predetermined. Then, the mask 194 is illuminated with the illumination light 193. On a quartz substrate of the mask 194, there is formed, with chromium or the like, a pattern which corresponds to a minute pattern to be formed on the wafer 198. The pattern on the mask 194 is in such a size that is obtained by multiplying the size of the minute pattern on the wafer 198 by a reciprocal number of the projection magnification of the projection optical system 196 (for example, two, four or five times). The illumination light 193 is diffracted through the minute pattern of the mask 194 to become the object-side exposure light 195. The projection optical system 196 converts the object-side exposure light 195 into the image-side exposure light 197, which forms an image of the minute pattern of the mask 194 on the surface of the wafer 198 at the above-stated projection magnification and with sufficiently small aberration. As shown in an enlarged view at the lower left part of FIG. 39, the image-side exposure light 197 converges on the wafer 198 at a predetermined numerical aperture NA (=sin xcex8) to form the image of the minute pattern on the wafer 198. When the minute pattern is to be formed on a plurality of different areas (shot areas which become one or a plurality of chips) of the wafer 198 one after another, the substrate stage 199 is moved stepwise along the image plane of the projection optical system 196 in such a way as to vary the position of the wafer 198 relative to the projection optical system 196.
The projection exposure apparatus which uses the excimer later as a light source and is most popularly in use these days has a high projection resolving power. However, it is technically difficult to form a pattern image of a line width not greater than, say, 0.15 um with the projection exposure apparatus.
The resolution of the projection optical system 196 is limited by a trade-off between the optical resolution and the depth of focus which depend on the exposure wavelength (wavelength of light used for exposure). The resolution R and the depth of focus DOF of the projection exposure apparatus are expressed by the following Rayleigh""s formulas (1) and (2):
R=k1xc2x7(xcex/NA)xe2x80x83xe2x80x83(1)
DOF=k2xc2x7(xcex/NA2)xe2x80x83xe2x80x83(2)
In these formulas, xe2x80x9cxcexxe2x80x9d represents the exposure wavelength, xe2x80x9cNAxe2x80x9d represents a numerical aperture on the image side indicating the brightness of the projection optical system 196, and k1 and k2 represent constants which are normally between 0.5 and 0.7 and are determined by the characteristic of a developing process on the wafer 198, etc. According to the formulas (1) and (2), in order to make the value of the resolution R smaller for a higher resolution, it is conceivable to increase the numerical aperture NA. However, in actually making an exposure, it is difficult to increase the numerical aperture NA more than a certain extent, because the depth of focus DOF of the projection optical system 196 must be kept above a certain value. In order to increase the resolution, therefore, it is necessary to make the exposure wavelength xcex smaller for shorter wavelength after all.
The attempt to shorten the exposure wavelength, however, encounters a serious problem. The problem lies in that it becomes hardly possible to find any optical material for lenses which form the projection optical system 196. Almost all optical materials have their transmission factors near to xe2x80x9c0xe2x80x9d in the far ultraviolet region. Although there is a fused quartz material which is manufactured by a special method for an exposure apparatus to have an exposure wavelength of about 248 nm, the transmission factor of the fused quartz also abruptly drops for the exposure wavelength not greater than 193 nm. It is thus extremely difficult to develop any optical material practically usable for an exposure wavelength not greater than 150 nm required for a minute pattern of line width not greater than 0.15 xcexcm. Besides, any optical material to be used within the far ultraviolet region is required to satisfy a plurality of conditions relative to durability, uniform refractive index, optical strain, workability, etc., in addition to the transmission factor. Accordingly, it is doubted whether there exits any practically-usable optical material.
Use of some of optical materials, such as CaF2, MgF2, etc., that have a fairly good transmission factor even for the wavelength of 150 nm or thereabout has recently come to be considered. However, the exposure wavelength is of course preferred to be longer.
As described above, in order to form on the wafer a pattern of a line width not greater than 0.15 xcexcm, the conventional projection exposure method and apparatus necessitate the exposure wavelength to be shortened down to a wavelength not greater than 150 nm. For this wavelength region, however, no practically-usable optical material is obtainable at present. Therefore, it has been hardly possible to form on the wafer any pattern of a line width not greater than 0.15 xcexcm.
With the above background taken into consideration, in the art of obtaining a minutely designed semiconductor device by photolithography, a phase shifting art for attaining a further improved resolution is attracting attention. The phase shifting art is developed to improve the light intensity profile by giving a phase shift to light passing through a mask.
The prior art for phase shifting is disclosed in Japanese Laid-Open Patent Application No. SHO 58-173744, an article entitled xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d by Marc D. Levenson, et al., IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, December 1982, PP. 1828-1836, and another article entitled xe2x80x9cThe Phase-shifting Mask II: Imaging Simulations and Submicrometer Resist Exposuresxe2x80x9d by Marc D. Levenson, et al., IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, June 1984, PP. 753-763.
Further, in Japanese Patent Publication No. SHO 62-50811, there is disclosed a phase shifting mask arranged to have a predetermined pattern composed of transparent parts and opaque parts. In the phase shifting mask, a phase member is provided on at least one of two transparent parts sandwiching the opaque part on its two sides, thereby bringing about a phase shift at the transparent parts on the two sides of the opaque part.
A phase shifting art called the Levenson type among known phase shifting arts is described below with reference to FIGS. 40(a) and 40(b).
In forming a line-and-space pattern on a wafer, an ordinary conventional mask is arranged as shown in FIG. 40(a). Referring to FIG. 40(a), light blocking parts 10 are formed with a light blocking material, such as Cr (chromium), some other metal or some metal oxide, on a transparent substrate 1 such as a quartz substrate. A line-and-space repeating pattern is formed by the light blocking parts 10, so that a mask for exposure is made. An intensity distribution of light passing through the mask for exposure is set such that, as indicated by reference symbol Al in FIG. 40(a), in an ideal state, the light is zero at each of the light blocking parts 10 and passes at the other parts, i.e., at light-transmitting parts 12a and 12b. 
Considering one light-transmitting part 12a, the transmitted light to be given to a material to be exposed is caused to have a light intensity distribution A2 by the diffraction of light, etc. As shown in 40(a), the light intensity distribution A2 has hill-like maximum parts on its two skirt sides. At the other light-transmitting part 12b, the transmitted light is caused to have a light intensity distribution A2xe2x80x2 as indicated by a one-dot-chain line. When light fluxes passing through the two light-transmitting parts 12a and 12b join each other to have a light intensity distribution A3, their light intensity distributions lose sharpness as shown by the light intensity distribution A3. Then, the image of the pattern is caused to blur by the diffraction of light. As a result, it becomes impossible to make a sharp exposure. On the other hand, if a phase shift part 11a which is called a shifter and is made of SiO2 or a resist is provided on every other light-transmitting parts 12a and 12b of the above-stated repeating pattern in a manner as shown in FIG. 40(b), the blur of image due to diffraction of light is cancelled by the inversion of phase, so that the image of the pattern can be sharply transferred, thereby improving the resolving power and the depth of focus.
More specifically, as shown in FIG. 40(b), when the phase shift part 11a is formed on one light-transmitting part 12a to give a phase shift of, for example, 180 degrees, a light portion passing through the phase shift part 11a is inverted as indicated by reference symbol B1. Meanwhile, another light portion passing through the adjoining light-transmitting part 12b is not inverted as there is no phase shift part. The phase-inverted light portion and the adjoining light portion to be given to the material to be exposed then act to offset each other at a skirt part of the light intensity distribution curve as indicated by reference symbol B2 in FIG. 40(b). As a result, the distribution of light applied to the material to be exposed takes an ideally sharp shape as indicated by reference symbol B3 in FIG. 40(b).
In the above-stated case, in order to ensure a maximum effect of the phase shifting arrangement, it is most advantageous to have the phases of the adjacent light portions inverted 180 degrees relative to each other. For this purpose, the phase shift part 11a is formed with a film of a thickness D, which can be expressed as follows:   D  =      λ          2      ⁢              (                  n          -          1                )            
where xe2x80x9cnxe2x80x9d represents the refractive index of a material with which the phase shift part is formed, and xcex represents the exposure wavelength.
The phase shifting mask arranged as described above to shift the phases of light portions passing through the adjacent light-transmitting parts relative to each other (ideally, to invert 180 degrees ) is called a spatial frequency modulating type (or a Levenson type). Phase shifting masks of other types include, for example, an edge emphasizing type, a light-blocking-effect emphasizing type, etc. Although a type having no light-blocking parts (called a chromeless type or the like) is included in the above various types of phase shifting masks, in each of those types, a phase shift part is provided to shift the phase of light passing through one part of the mask relative to the phase of light passing through an adjacent part of the mask.
In Japanese Laid-Open Patent Application No. HEI 6-83032, there is disclosed a multiple exposure method using two phase shifting masks. According to this method, an exposure is made by using a first phase shifting mask in which light-transmitting parts and phase shift parts are alternately formed, and another exposure is made by using a second phase shifting mask in which light-transmitting parts and phase shift parts are formed in an arrangement inverse to the arrangement of the first phase shifting mask.
Further, in Japanese Laid-Open Patent Application No. HEI 7-50243, there is disclosed another exposure method. In that method, phase shifters are provided in predetermined positions of light-transmitting areas of a mask in which a plurality of patterns are formed at intervals which are about the same as or shorter than the exposure wavelength, so as to bring about a phase shift between one passing light portion and another when a sample is exposed with projected images of the plurality of patterns. In that instance, among patterns to be formed consecutively by exposures, at least one of the consecutive patterns is divided into a pattern portion for which the boundary of phase inversion occurs in the transmitted light passing through the corresponding light-transmitting area and the other pattern portion, and the divided pattern portions are respectively formed on different masks. Then, exposures are made while adjusting the relative positions of the two masks.
However, the circuit pattern of an integrated circuit to be actually transferred to a semiconductor wafer is designed in a complicated manner and is not simply composed of vertical and horizontal pattern lines. In cases where a plurality of patterns are included in a closed pattern as shown in FIG. 41 and where a pattern includes a U-shaped pattern as shown in FIG. 42, the conventional multiple exposure method using the phase shifting mask often fails to effectively carry out the exposure.
It is an object of the invention to provide masks, an exposure method and an exposure apparatus by which an exposure can be effectively carried out by using phase shifting masks even for a complicated pattern.
To attain the above object, in accordance with an aspect of the invention, there are provided a pair of masks, i.e., a first mask and a second mask, used for a multiple exposure to expose a substrate with a desired pattern, wherein the first mask and the second mask have respective phase shift areas formed correspondingly with the desired pattern and having respective different phase shifting effects.
More specifically, the phase shift areas of the first and second masks are arranged to cause the phase of light passing there to be shifted 180 degrees from the phase of light passing light-transmitting parts of the first and second masks other than the phase shift areas.
The phase shift area of the first mask is set in such a way as to resolve minute lines in one direction, and the phase shift area of the second mask is set in such a way as to resolve minute lines in another direction perpendicular to the one direction.
The pattern on each mask is set to be used for an exposure with light of a wavelength not greater than 250 nm.
In accordance with another aspect of the invention, there is provided an exposure method for making a multiple exposure by using the first and second masks. The term xe2x80x9cmultiple exposurexe2x80x9d as used herein means an exposure process to be performed a plurality of times without performing a developing action on a resist between one exposure and another exposure on one and the same area of the resist.
In accordance with a further aspect of the invention, there is provided an exposure method for exposing a resist in a certain pattern, comprising a first exposure step of exposing the resist through a first mask, and a second exposure step of exposing the resist through a second mask, wherein the first exposure step and the second exposure step are either serially executed in the mentioned order or in the reverse order or simultaneously executed, and, if the first exposure step and the second exposure step are serially executed, no developing process is performed on the resist between the first exposure step and the second exposure step, wherein each of the first mask and the second mask has a light-blocking part and a non-light-blocking part, and a shape of the non-light-blocking part of the first mask and a shape of the non-light-blocking part of the second mask are substantially the same as a shape of the pattern, wherein the first mask is arranged such that predetermined areas, within the non-light-blocking part thereof, which are adjacent to each other across the light-blocking part thereof at least in a first direction give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas, and wherein the second mask is arranged such that predetermined areas, within the non-light-blocking part thereof, which are adjacent to each other across the light-blocking part thereof at least in a second direction different from the first direction give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas.
In accordance with a further aspect of the invention, there is provided an exposure method for exposing a resist in a certain pattern, comprising a first exposure step of exposing the resist through a first mask, and a second exposure step of exposing the resist through a second mask, wherein the first exposure step and the second exposure step are either serially executed in the mentioned order or in the reverse order or simultaneously executed, and, if the first exposure step and the second exposure step are serially executed, no developing process is performed on the resist between the first exposure step and the second exposure step, wherein each of the first mask and the second mask has a light-blocking part and a non-light-blocking part, and a shape of the non-light-blocking part of the first mask and a shape of the non-light-blocking part of the second mask are substantially the same as a shape of the pattern, wherein the first mask is arranged such that, within the non-light-blocking part thereof, a boundary of phases is formed along a first direction and predetermined areas which are adjacent to each other across the boundary give a phase shift of substantially an odd-number of times as much as n between light portions propagating through the respective predetermined areas, and wherein the second mask is arranged such that, within the non-light-blocking part thereof, a boundary of phases is formed along a second direction different from the first direction and predetermined areas which are adjacent to each other across the boundary give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas.
In accordance with a further aspect of the invention, there is provided an exposure method for exposing a resist in a certain pattern, comprising a first exposure step of exposing the resist through a first mask, and a second exposure step of exposing the resist through a second mask, wherein the first exposure step and the second exposure step are either serially executed in the mentioned order or in the reverse order or simultaneously executed, and, if the first exposure step and the second exposure step are serially executed, no developing process is performed on the resist between the first exposure step and the second exposure step, wherein each of the first mask and the second mask has a light-blocking part and a non-light-blocking part, and a shape of the non-light-blocking part of the first mask and a shape of the non-light-blocking part of the second mask are substantially the same as a shape of the pattern, wherein the first mask is arranged such that predetermined areas, within the non-light-blocking part thereof, which are adjacent to each other across the light-blocking part thereof at least in a first direction give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas, and such that, within the non-light-blocking part thereof, a boundary of phases is formed along a third direction and predetermined areas which are adjacent to each other across the boundary give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas, and wherein the second mask is arranged such that predetermined areas, within the non-light-blocking part thereof, which are adjacent to each other across the light-blocking part thereof at least in a second direction different from the first direction give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas, and such that, within the non-light-blocking part thereof, a boundary of phases is formed along a fourth direction different from the third direction and predetermined areas which are adjacent to each other across the boundary give a phase shift of substantially an odd-number of times as much as xcfx80 between light portions propagating through the respective predetermined areas.
In accordance with a further aspect of the invention, there is provided an exposure method for exposing a resist in a certain pattern, comprising a first exposure step of exposing the resist through a first mask, and a second exposure step of exposing the resist through a second mask, wherein the first exposure step and the second exposure step are either serially executed in the mentioned order or in the reverse order or simultaneously executed, and, if the first exposure step and the second exposure step are serially executed, no developing process is performed on the resist between the first exposure step and the second exposure step, wherein each of the first mask and the second mask has a light-blocking part and a non-light-blocking part, and a shape of the non-light-blocking part of the first mask and a shape of the non-light-blocking part of the second mask are substantially the same as a shape of the pattern, wherein the first mask gives a phase shift of an odd-number of times as much as n between light portions propagating through different areas of the non-light-blocking part in such a way as to have the resist exposed with a part of the pattern, and wherein the second mask gives a phase shift of an odd-number of times as much as xcfx80 between light portions propagating through different areas of the non-light-blocking part in such a way as to have the resist exposed with another part of the pattern.
In accordance with a further aspect of the invention, there is provided an exposure apparatus which has an exposure mode in which a multiple exposure is performed by using the first and second masks.
In accordance with a further aspect of the invention, there is provided a method for manufacturing a device, in which the device is manufactured by exposing a wafer with a pattern for the device according to the above exposure method and, after that, carrying out a developing process on the exposed wafer.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.